"but thy eternal summer shall not fade"? – Yahoo! Answers

The youth and good looks of the young person that Shakespeare addresses in this sonnet are like a summer day. Except that summer days are sometimes too hot and sweaty or sometimes cloudy, and they don’t last all that long. But the summer of his admired one, such youth and "fair" looks, are ideal and they won’t fade because the poet has captured and preserved them in these lines. "Thy eternal summer shall not fade . . . / When in eternal lines to time thou growest."

Some biographers say Shakespeare was writing these lines to a young seventeen-year-old man, perhaps an actor in his company or the son of one of his financial backers; some think he is writing a poem FOR this young man to present to his beloved. Some even suggest he is writing to a "dark lady" with whom both he and the young man are having an affair. Nobody knows for sure. We’re fairly certain he was NOT writing to his wife Anne, who was several years older than Shakespeare and far away in Stratford taking care of their children.

So what does not fade?

Somebody’s youthful good looks–a lovely, temperate summer–that the poet has made eternal. We just don’t know whose!